1. Field of the Invention and Related Art Statement
The present invention relates to a circuit for driving an ultrasonic transducer, and more particularly relates to a circuit for driving an ultrasonic transducer for use in surgical operations.
There have been developed various kinds of devices using ultrasonic transducers such as ultrasonic surgical knives, ultrasonic working tools, ultrasonic atomizes, ultrasonic bonding machines and ultrasonic welding machines. In these ultrasonic devices, in order to improve the efficiency it is desired to drive the ultrasonic transducer at a resonance frequency. However, since the resonance frequency is changed in accordance with temperature variations, it is rather difficult to vibrate the ultrasonic transducer always at the resonance frequency, when there is not provided any means for compensating for the variation of the resonance frequency. Furthermore, the resonance frequency of the ultrasonic transducer is changed in accordance with the condition of the load applied thereto. Therefore, it is required to adjust or change the frequency of the driving signal in accordance with the variation of the resonance frequency of the ultrasonic transducer.
Several solutions for satisfying the above mentioned requirement have been proposed in, for instance U.S. Patent Nos. 4,275,363, 4,587,958, 4,724,401 and 4,754,186. In these known ultrasonic transducer driving circuits, there is provided a phase lock loop (PLL) and the frequency of the signal for driving the ultrasonic transducer is automatically controlled to follow the varying resonance frequency of the ultrasonic transducer. However, the inventor present has confirmed that the known ultrasonic transducer driving circuits including a PLL have a serious drawback which will be explained hereinbelow.
FIG. 1 shows an equivalent circuit of the piezoelectric type ultrasonic transducer. The ultrasonic transducer 1 is expressed by a parallel circuit of series-connected resistor R, inductor L and capacitance C and a damping capacitance C.sub.d. In a practical circuit, in order to cancel the effect of the damping capacitance C.sub.d, a compensating inductor L.sub.d is connected in parallel with said parallel circuit of the ultrasonic transducer 1. In such a circuit, the frequency characteristic of a phase difference .DELTA..theta. between the driving voltage and the driving current can be represented by a curve shown in FIG. 2A, and the frequency characteristic of an impedance .vertline.Z.vertline. viewed in the direction shown by an arrow A in FIG. 1 is illustrated in FIG. 2B. As illustrated in FIGS. 2A and 2B, the phase difference .DELTA..theta. becomes zero at a resonance frequency f.sub.r as well as antiresonance frequencies f.sub.1 and f.sub.2, these antiresonance frequencies being positioned on respective sides of the resonance frequency f.sub.r, and the impedance .vertline.Z.vertline. becomes minimum at the resonance frequency f.sub.r and becomes maximum at the antiresonance frequencies f.sub.1 and f.sub.2. In the known driving circuit, the phase difference .DELTA..theta. between the driving voltage and the driving current is detected to adjust the driving frequency into the resonance frequency f.sub.r through the feedback control of the PLL. As can be understood from the curves shown in FIGS. 2A and 2B, the phase difference .DELTA..theta. becomes zero not only at the desired resonance frequency f.sub.r, but also at the antiresonance frequencies f.sub.1 and f.sub.2, so that the feedback control of the PLL in which the frequency of the driving signal is adjusted to follow the resonance frequency of the ultrasonic transducer is effective only within the frequency range between the two antiresonance frequencies f.sub.1 and f.sub.2, and if the vibrating frequency of the ultrasonic transducer decreases lower than the antiresonance frequency f.sub.1 or increases higher than the antiresonance frequency f.sub.2, the feedback control of the PLL could not be performed correctly and the driving frequency would further decrease or increase continuously. Particularly, in the time of starting the vibration, the driving signal could not be easily locked with the vibration of the ultrasonic transducer at the desired resonance frequency f.sub.r.
In order to mitigate the above mentioned drawback, it has been known to restrict the frequency control range of the PLL with the aid of a limiter. In this case, since the limiter has to be set or designed very precisely, the circuit construction is liable to be complicated, so that such a driving circuit has not been actually realized.
In the above mentioned U.S. Pat. No. 4,275,363, there has been proposed to provide a sweep circuit by means of which a control voltage applied to a control terminal of a voltage controlled oscillator (VCO) in the PLL is changed in a monotonous manner so that the oscillation frequency of the VCO is changed within a predetermined range, and when the resonance point is detected, the sweep operation is stopped and the PLL is set into the feedback control mode. However, in this known driving circuit, the output voltage from the sweep circuit is applied to the VCO at the transient between the sweep control mode and the feedback control mode and a finite voltage is applied to the VCO as an offset voltage. This offset voltage affects the loop characteristics of the PLL, and the PLL could not operate correctly.
In the U.S. Pat. No. 4,754,186, there is proposed another method of locking the oscillation frequency of the VCO to the resonance frequency of the transducer. In this known method, when the VCO oscillates at a frequency lower than the lower antiresonance frequency f.sub.1, a pulse signal is added to the feedback signal in the PLL so that the oscillation frequency of the VCO is increased and is locked into the desired resonance frequency f.sub.r. However, in this known method, since the locking operation is dependent upon the loop characteristics of the PLL, it is quite difficult to positively lock the oscillation frequency of the VCO into the resonance frequency f.sub.r of the ultrasonic transducer.
It should be noted that the above described problem occurs not only during the starting period but also during the usual operation. That is to say, in the case that the ultrasonic transducer is used in a surgical knife, when a very large load is applied to the ultrasonic transducer, the ultrasonic transducer could not be vibrated at the resonance frequency f.sub.r and the driving frequency might be out of the automatic resonance point following range. Then, it is necessary to effect the lock-in operation again.